Cutting fluids have been used in machining processes for many years to increase lubricity by spraying the coolant into the machining zone directly on the cutting tool and the part. This has the effect of decreasing the friction between the chip and the tool, which in turn decreases the tool temperature, increases tool life, and improves the part quality. These benefits come with certain drawbacks. In high-volume machining operations, at least 16% of the machining cost is associated with the procurement, maintenance, and disposal of cutting fluids. This cost does not account for the health risks that workers are exposed to when using these fluids. Contact with cutting fluids or their mists can cause maladies such as dermatitis and respiratory diseases. Some additives in cutting fluids may be carcinogenic.
In recent years, because of these problems, the high-volume machining industry has moved toward dry machining to reduce or eliminate the use of cutting fluids. However, dry machining increases fossil fuel consumption and energy costs because larger and more powerful machines are required to process the less lubricious material. Dry machining also increases the per part costs by consuming more cutting tools and requiring more machining time. The problem is exacerbated when machining titanium and other low thermal conductivity materials since the heat produced at the tool-chip interface is not readily conducted away from the interface by the material itself. Further, dry machining is not feasible for relatively small shop sites, where the capital for new machines is often not available.
Past research efforts and patents have focused on internally or externally cooling the cutting tool holder, spraying liquid nitrogen into the machining zone, using high-pressure coolants, and the integration of a cap-like reservoir on top of the cutting tool insert that is cooled by liquid nitrogen.
Internally and externally cooling the cutting tool has been experimentally tested using heat pipes. Some degree of cooling was achieved, but the heat transfer efficiency of the design is very low. No measurements of cutting tool flank wear reduction were made, possibly due to the poor performance of the system on the bench-top.
Spraying a jet of liquid nitrogen into the machining zone has proven to be an effective means to cool the cutting tool, but a large amount of liquid nitrogen is used in the process owing to the comparatively low heat transfer effectiveness of this approach. This increases the environmental impact of the liquid nitrogen jet for two reasons. First, a ventilation system is required to remove the large amounts of nitrogen vapor created during the cooling process. Second, the electrical power needed to produce the large amount of liquid nitrogen used by this cooling method requires more fossil fuel and, correspondingly, increases pollution.
The use of high pressure jets of coolant to reduce the tool wear has also been investigated. Such an approach can effectively decrease tool wear, but has several drawbacks. First, the jets require pressurized coolant using a large compressor that consumes electrical power, which increases the cost and environmental impact of the process. Second, the jets need to be applied to particular locations on the cutting tool insert. This requires accurate and repeatable positioning of the small diameter, high-pressure jet relative to the cutting tool insert. This approach is not feasible in a production environment, where the overhead associated with managing the high-pressure liquid jet quickly drives up the machining time and the costs. Third, the high-pressure jets require liquid flow rates that are one to three orders of magnitude larger than indirectly cooling the tool-chip interface as disclosed herein. This fact dramatically increases the cost and the environmental impact of using high pressure jets.
Another approach involves the integration of a cap-like reservoir cooled with liquid nitrogen on top of the cutting tool insert, and this has been shown to decrease the tool wear. This arrangement has a relatively low heat transfer efficiency however, and as a result the necessary flow rates are two to three orders of magnitude larger than the method that is disclosed herein. Because the reservoir is located on top of the cutting tool insert, the device is difficult to use in a production environment. In order to index or change the insert, the operator needs to remove and reattach the reservoir, which is at cryogenic temperatures. These operations require special training, increasing costs, and increases the health risks to operators. For these reasons, it is unlikely that such a system would be used in a production environment.